A phased array antenna is composed of multiple radiating antenna elements, individual element control circuits, a signal distribution network, signal control circuitry, a power supply, and a mechanical support structure. The total gain, effective isotropic radiated power and scanning and side lobe requirements of the antenna are directly related to the number of elements in the antenna aperture, the element spacing, and the performance of the elements and element electronics. In many applications, thousands of independent element/control circuits are required to achieve a desired antenna performance. A typical phased array antenna includes independent electronic packages for the radiating elements and control circuits that are interconnected through an external distribution network. FIG. 1 shows a schematic of a typical transmit phased array antenna which includes an input, distribution network, element electronics and radiators.
As the antenna operating frequency increases, the required spacing between radiating elements decreases and it becomes difficult to physically configure the control electronics and interconnects within the increasingly tight element spacing. Relaxing the tight element spacing will degrade the beam scanning performance, but adequately providing multiple interconnects requires stringent manufacturing and assembly tolerances which increase system complexity and cost. Consequently, the performance and cost of the phased array antenna depends primarily on module packaging and distribution network interconnects. Multiple beam applications further complicate this problem by requiring more electronic components and interconnects within the same antenna volume.
Phased array packaging architectures can be divided into tile (i.e., coplanar) and brick (i.e., in-line) styles. FIG. 2 shows a typical tile-type architecture which exhibits components that are co-planar with the antenna aperture and which are assembled together as tiles. FIG. 3 shows a typical brick-type architecture which uses in-line components that are perpendicular to the antenna aperture and are assembled together similar to bricks.
The assignee of the present application, The Boeing Company, has been a leading innovator in phased array module/element packaging technology. The Boeing Company has designed, developed and delivered many phased arrays which use tile, brick and hybrid techniques to fabricate radiator modules and/or distribution networks. The RF distribution network which provides electromagnetic wave EM energy to each of the phased array modules can be delivered in what is called “series” or “parallel”. Series distribution networks are often limited in instantaneous bandwidth because of the various delays which the EM wave signal experiences during the distribution. Parallel networks, however, provide “equal delay” to each of the modules, which allows wide instantaneous bandwidth. However, parallel distribution increases in difficulty with a large number of radiator modules. The most common method to deliver equal delay to a group of phased array modules is a “corporate” distribution network. The corporate distribution network uses binary signal splitters to deliver equally delayed signals to 2n modules. This type of distribution lends itself well to the tile array architecture that has been used extensively throughout industry.
The use of a corporate network in a tile architecture is limited by the module spacing. It becomes increasingly more difficult to distribute EM wave energy, DC power signals, and logic signals with tightly-packed modules of wide-angle beam scanning arrays at higher operating frequencies. Because the cost of RF power also increases with operating frequency, designers try to limit distribution losses by using low-loss transmission media. The lowest loss medium used is an air filled rectangular waveguide. However, such a waveguide requires a large volume and is not easily routed to individual sites (i.e., antenna modules). Stripline conductors, depending on material parameters and dimensions, can exhibit as much as 5-10 times the amount of loss per unit length of waveguide as an air filled rectangular waveguide. However, a stripline waveguide is very compact and readily able to distribute RF energy to tightly-packed modules (i.e., radiating elements) that are separated by only a very small amount of spacing.
Air filled waveguides can be used exclusively in a series network to feed tightly packed antenna modules. Each air filled length of waveguide uses a series of slots in what is referred to as a “rail”. The electrical length between the slots in a rail changes with the operating frequency. If the rail is used to form an antenna beam, the change in electrical length between slots causes the beam to shift or “squint” away from the intended angle as the operating frequency changes. As the number of slots in the rail is increased, the beam squint becomes more pronounced, thus reducing the instantaneous bandwidth even further. The slots in a rail also tend to interact with each other and make rail designs more difficult and complex. If the slots were isolated from each other, then the length of each slot needed for the desired coupling levels could be more easily determined. A rail also achieves its desired phase and amplitude distribution at a single center frequency and quickly degrades as the operating frequency deviates away from the center frequency.
For a phased array antenna, the phase errors introduced by series distribution networks can be adjusted for in the antenna module using phase shifters. To accomplish the adjustment or calibration, a priori knowledge of the instantaneous operating frequency is required. A look-up table is used to correct for the beam squint at various frequency points along the operating bandwidth of the array. The length of the rail determines the number of steps or increments required to adequately adjust the phase shifters. Longer rails cause more beam squint and narrower instantaneous bandwidth, which means that more frequency increments are required to calibrate the numerous antenna modules of the antenna.
A particularly challenging problem that The Boeing Company has been faced with, and which the antenna and method of the present invention overcomes, is developing a wide-beam scanning, Q-band phased array antenna capable of operating at 44 GHz for MILSTAR communications. The MILSTAR communication protocol uses narrowband bursts of information frequency hopping over the 2 GHz bandwidth of operation. However, the use of a series fed waveguide and the differing beam squints requires knowledge of the next beam hopping frequency so that the appropriate delay can be obtained from the look-up table and applied to the phase shifters. Without such knowledge of the next beam hopping frequency, the series fed beam rail squints cannot be accurately determined. For security reasons, it is desirable for a phased array antenna system to not require specific frequency information for operation but instead to be able to operate over the entire bandwidth as a passive device. A new form of corporate feed waveguide network is therefore required which allows very tight module spacing, but which still does not require individual series led rail beams squints to be calculated to maintain calibration of all of the individual module elements of the antenna.